Rebirth of X-ray Emission from the Born-Again Planetary Nebula A 30

The planetary nebula (PN) A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using HST images we have detected the angular expansion of these knots and derived an age of 850+280-150 yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton observations of A30. The X-ray emission from A30 can be separated into two components: a point-source at the central star and diffuse emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters of the central star of A 30 using a non-LTE model fit to its optical and UV spectrum. The spatial distribution and spectral properties of the diffuse X-ray emission is suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass-loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta has also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A 30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible.Comment: 29 pages, 11 figures, 4 tables; accepted for publication by Ap

Cratering on Gaspra

Galileo flyby images of 951 Gaspra show a crater population dominated by fresh craters several hundreds meters in diameter and smaller. They must represent production population because their spatial density is low (few overlaps) and because degraded craters are underabundant; equilibrium may be attained at diameters near to or below the resolution limit of the best image. We have counted, measured, and classified craters from highest resolution, "high phase" image, which shows >600 craters in 90 km_2. The differential population index (0.2 - 0.6 km) for the fresh, obvious crater is very "steep" (-4.3 +- 0.3). It probably reflects the index of asteroidal projectiles; it is much steeper than the theoretical valueof -3.5 for collisional equilibrium. Gaspra's crater population differs from that observed on Phobos but resembles those observed on the Moon and Mars at these sizes (consistent also with the near-Earth asteroid population). Gaspra's fresh craters are superposed on a landscape that appears "smoothed" at a vertical scale of hundreds of meters. Some "soft", subdued crater-like features, commonly >500m across, are visible. Some of these are associated with the linea grooves on Gaspra and may be endogenic features. Many others are probably pre-existing impact craters deeply blanketed or otherwise much degraded. Gaspra's largest-scale shape is highly irregular, perhaps faceted. The biggest facet exceeds the largest crater (relative to body radius) ever observed on a satellite or expected from collisional fragmentation models. Facets cannot be successive crater-forming impacts; later scars would have destroyed earlier ones. Far-encounter images show a more lumpy that faceted visage of Gaspra; the two craters are 3 km in diameter, not even half the radius of Gaspra. We expect that Gaspra was created by collisional fragmentation of a larger parent body

New near-aphelion light curves of Comet 2P/Encke

We present new, near-aphelion, time series of photometry of Comet 2P/ Encke in Cousins-R band. With these light curves we find that the dominant, synodic rotational periodicity is either P0 = 11.079 ± 0.009 h or 2P0 = 22.158 ± 0.012 h. This is in contrast to data from the 1980s published by others that are consistent with 15.08- and 22.6-h periods. Those periods do not satisfy our phased light curves, and also the 1980s data are not easily reconciled with our periods. This could be due to P/Encke having non-principal axis rotation or due to a drift in the rotation period caused by outgassing torques. We observed the comet at five epochs: July, August, September, and October 2001, and September 2002, and the comet was at times intrinsically brighter than expected for a bare nucleus, due to an apparent contribution from an unresolved coma. Three-quarters of the data were obtained in the second and fifth epochs, and we analyzed these two time series using both the phase-dispersion minimization and "WindowCLEAN" techniques. At both epochs and with both techniques strong periodicities were found near frequencies f0 = 2.16 d^-1 and f1 = 4.35 d^-1. By then using visual inspection of the phased light curves to corroborate these frequencies, and by using the data from the other three epochs to properly align light curve features, we were able to derive P0 and 2P0 as the only solutions that satisfy all our observations. The periodicity due to f1 is clearly seen in our data, but we cannot tell from our data alone whether it is a manifestation of the nucleus's shape, non-principal axis rotation, or both. © 2004 Elsevier Inc. All rights reserved

The shape of Io from Galileo limb measurements

Galileo CCD images of the limb of Io provide improved data for determining the shape of this synchronously rotating satellite. The best ellipsoidal fit is within 0.3 km of the best equilibrium fit of 1829.7, 1819.2, 1815.8 km The shape is consistent with substantial mass concentration in a core and with gravity measurements from tracking of the Galileo spacecraft. The surface of Io is largely plains and isolated peaks, with little long-wavelength topography over 1 km in amplitude

Control Networks on the Galilean Satellites: Solutions for Size and Shape

A control network is a series of identifiable points on a surface and a table of their coordniates (latitude, longitude, radius). To create a planetary control network, points (usually craters) are identified on pixtures and their image coordinates are measured in pixels. Each control point must be measured on two or more images. The navigation team supplies discrete spacecraft positions and approximate camera-pointing angles. An analytical triangulation program is used to compute the coordinates of the control points and to improve the camera-pointing angles. A control network supports the compilation of maps of a particular reregion or an entire body. The maps may abe planimetric or topographic. A reference surface is used to approximate the shape of a body or to measure elevations. The reference surface is usually a sphere or spheroid, so the map can be displayd in many popular projections. Ideally, the reference surfaces for the Galilean satellites should beellipsoids, because they are in synchronous orbits and experience strong tidal forces. However, most popular projections such as Mercator, Lambert, and stereographic lose their elegant and convenient properties when the reference surface is an ellipsoid. Experiments were made to solve for the three axes as indipendent variables in the analytical triangulation. The results are of little use, as the control points are not uniformly distributed and the image resolutions vary greatly. However, the control networks can be used to study planetary shapes when combined with gravity data to constrain models of internal structure